Recent carbon dioxide (CO.sub.2) gas laser oscillation devices are capable of producing high output power and emitting good-quality laser beams, and are widely used in laser machining applications such as metal or nonmetal material cutting and metal material welding. Particularly, those carbon dioxide gas laser oscillation devices which are coupled to CNC (computerized numerical control) systems to provide CNC laser machining apparatus are in widespread use in the application in which workpieces are cut to complex shapes at high speed with high precision.
One conventional carbon dioxide (CO.sub.2) gas laser oscillation device will hereinafter be described with reference to the drawings.
FIG. 6 of the accompanying drawings shows a conventional carbon dioxide (CO.sub.2) gas laser oscillation device in its entirety. The laser oscillation device includes a discharge tube 31 combined with an optical resonator which comprises an output coupling mirror 32 and a fully reflecting mirror 33 that are connected to respective opposite ends of the discharge tube 31. Metal electrodes 34, 35, are attached to outer peripheral surfaces of the discharge tube 31, the metal electrode 34 being grounded and the metal electrode 35 being connected to a high-frequency power supply 36. The high-frequency power supply 36 applies a high-frequency voltage between the metal electrodes 34, 35 to produce a glow discharge in the discharge tube 31 for laser excitation. A laser beam is generated along an optical axis 43 in the discharge tube 31, and is emitted out of the discharge tube 31 from the output coupling mirror 32 along an optical axis 44.
Before the gas laser oscillation device is started, the entire device is evacuated by a vacuum pump 42, and a valve 41 is opened to introduce a laser gas at a given rate from a gas container 40 into the device until the pressure of the gas in the device reaches a predetermined level. The device is continuously evacuated by the vacuum pump 42 and the laser gas is continuously supplied through the valve 41 so that the laser gas is partly replaced with a fresh gas continuously while the gas pressure in the device is being kept at the predetermined level. In this manner, the interior of the device is prevented from being contaminated by the laser gas.
In FIG. 6, a blower 39 circulates the laser gas in the device in order to cool the laser gas. In the carbon dioxide (CO.sub.2) gas laser, about 20% of the supplied electric energy is converted into a laser beam, whereas the rest of the applied electric energy is consumed to heat the laser gas. Since the gain of laser oscillation is theoretically proportional to the -(3/2)th power of the absolute temperature T, it is necessary to forcibly cool the laser gas in order to increase the oscillation efficiency.
In the device shown in FIG. 6, the laser gas flows through the discharge tube 31 at a rate of about 100 m/sec. in the direction indicated by the arrow into a cooling unit 38. The cooling unit 38 mainly serves to remove the thermal energy produced by the electric discharge from the laser gas. The blower 39 then compresses the laser gas which has been cooled. The compressed laser gas is thereafter introduced into the discharge tube 31 through another cooling unit 37. The cooling unit 37 serves to remove the heat produced upon compression of the laser gas in the blower 39, before the laser gas is introduced again into the discharge tube 31. The cooling units 37, 38 will not be described in detail as they are well known in the art.
The blower 39 may be either a roots blower or a turbo blower. FIG. 7 of the accompanying drawings shows the structure of a conventional turbo blower for a laser. The turbo blower includes an impeller 1 mechanically coupled to a shaft 2 on which a rotor 3 is mounted. The rotor 3 and a stator 4 disposed therearound make up a high-frequency motor. The impeller 1 is rotated at a high speed of about 100,000 RPM by the high-frequency motor. Because of the high rotational speed of the impeller 1, the turbo blower is smaller in volume than the roots blower which rotates at lower speeds.
The shaft 2 is supported by a pair of roller bearings 5, 6 disposed one on each side of the high-frequency motor. The roller bearings 5, 6 are lubricated by grease in order to prevent contamination of the laser gas which would otherwise be caused by a lubricating oil mist.
The laser gas is drawn from the cooling unit 38 into the laser turbo blower as indicated by the arrow 8, and discharged from the laser turbo blower to the cooling unit 37 as indicated by the arrow 7.
The conventional laser oscillation device shown in FIGS. 6 and 7 has suffered the following drawbacks:
Inasmuch as the conventional laser turbo blower rotates at a high speed up to 100,000 RPM, the grease tends to be deteriorated and used up early. More specifically, the motor has an efficiency of about 75%, with the rest (25%) of the applied electric energy being converted into a heat loss. Therefore, if the motor has an output of 2 kw, then about 667 W is consumed as a heat loss. About 567 W of the heat loss is caused by the iron and copper losses of the stator 4, whereas about 100 W thereof is caused by the copper loss of the rotor 3.
When the rotor 3 is thus heated, its temperature goes up to 100.degree. C. or higher. The stator 4 can be cooled by a water cooling unit disposed around the stator 4, but the rotor 3 cannot be cooled by a water cooling unit as it rotates at high speed. The motor cannot be cooled by air since only the laser gas is present under a 0.1 atmospheric pressure in the motor.
Therefore, the heat generated by the rotor 3 is transmitted through the shaft 2 to the bearings 5, 6 fixed to the shaft 2, thus heating and increasing the temperature of the bearings 5, 6. The bearings 5, 6 operate without fail up to the temperature of about 80.degree. C. However, above the temperature of about 80.degree. C., the service life of the bearings is reduced 1/2 in each increment of 10.degree. C. because the lubricant of the bearings is deteriorated at higher temperatures.
If the bearings were continuously operated under the above condition, the bearings would be destroyed. Usually, therefore, the grease is replenished or the bearings are replaced at time intervals of 1000 hours. As a result, a large expenditure of labor has heretofore been necessary for maintenance.
The grease is vaporized by the heat transmitted to the bearings 5, 6, and the grease vapor is mixed with the laser gas. The mixed grease vapor contaminates the optical components, causing laser output power reductions and mode changes.
According to the conventional structure, the impeller 1 is positioned in the passage through which a large amount of laser gas is circulated. Therefore, the impeller 1 is always cooled, and temperature of the bearing 5 near the impeller 1 is not largely increased. However, since the bearing 6 remote from the impeller 1 is not cooled by the laser gas, the bearing 6 is directly affected by the temperature rise and very possibly may be damaged.